EGGSHELLS ARE SHAPED BY A PRECISE SPATIOTEMPORAL ARRANGEMENT OF SEQUENTIALLY DEPOSITED MACROMOLECULES

The avian eggshell is a composite bioceramic which is formed by a cont rolled interaction of an organic and an inorganic phase. The organic p hase contains, among other constituents, type X collagen and proteogly cans, mainly keratan and dermatan sulfate. Understanding the principle s governing the synthesis and temporo-spatial distribution of such mac romolecules, and their influence on the organization of the crystallin e phase, is an essential aspect of establishing the biological basis o f the quality of eggshell, both as an embryonic chamber and as a natur al food package. In the present study, we have examined the process of eggshell formation by immunohistochemistry, scanning electron microsc opy and energy dispersive X-ray microanalysis. Precise sites and timin g of secretion were established for the deposition of particular macro molecules. Type X collagen is detected at the very first moment of she ll membrane formation. The appearance of keratan sulfate coincides wit h the appearance of mammillae, while dermatan sulfate is deposited lat er, coincident with shell matrix deposition. We propose that keratan s ulfate, due to its precise localization, temporal appearance and calci um-binding affinity, relates to the maintenance of calcium reserve bod ies, the primary source of calcium for the embryo. On the other hand, dermatan sulfate may control crystal growth, resulting in a preferenti al orientation of calcite crystals within the palisade layer.